Mercury
Mercury is the closest planet to the Sun and the smallest planet in the Solar System. It is a rocky world covered in craters, with an extremely thin atmosphere and some of the most dramatic temperature changes of any planet.
Because Mercury orbits so close to the Sun, it moves faster than every other planet, racing through space at remarkable speed.
The Fastest Planet
Mercury completes one orbit around the Sun in just 88 Earth days. This short year makes it the fastest-moving planet in the Solar System. It travels at an average orbital speed of about 107,000 miles per hour (172,000 kilometers per hour).
Despite its fast orbit, Mercury rotates slowly. One rotation takes about 59 Earth days. Because of the relationship between its rotation and orbit, a full solar day—from one sunrise to the next—lasts about 176 Earth days.
A World of Extreme Temperatures
Mercury has an extremely thin atmosphere, called an exosphere, that cannot hold in heat or distribute it around the planet. As a result, temperatures vary sharply between day and night.
Daytime temperatures near the equator can reach about 800°F (430°C), while nighttime temperatures can fall to about -290°F (-180°C). Mercury's slow rotation contributes to these long periods of intense heating and cooling.
Surface Features
Mercury's surface resembles Earth's Moon. It is heavily cratered by billions of years of impacts and contains enormous basins formed early in the Solar System's history.
Major features include the Caloris Basin, large scarps or cliffs, ancient lava plains, and bright crater rays from more recent impacts. The scarps show that Mercury has contracted as its interior gradually cooled over time.
Key Facts About Mercury
Diameter: About 3,032 miles (4,879 kilometers)
Mass: About 0.055 times Earth's mass
Average Distance from the Sun: About 36 million miles (0.39 AU)
Year Length: About 88 Earth days
Rotation Period: About 59 Earth days
Solar Day: About 176 Earth days
Average Orbital Speed: About 107,000 mph (172,000 km/h)
A Giant Metal Core
Mercury has an unusually large iron-rich core. It extends across about 85% of the planet's radius, leaving a relatively thin rocky mantle and crust around it. Evidence suggests that parts of the core remain liquid.
Scientists are still investigating why Mercury has such a large core. Possible explanations include a giant early collision that removed part of its outer rock or processes that affected lighter material during the planet's formation.
Mercury's partly liquid core generates a global magnetic field. The field is weak compared with Earth's, but it is important because it interacts directly with the powerful solar wind near the Sun.
Ice Near the Poles
Even though Mercury is extremely hot in sunlight, water ice exists near its poles. Some polar craters are permanently shadowed, meaning sunlight never reaches their floors. These regions remain cold enough for ice to survive for very long periods.
Exploring Mercury
Mercury is difficult to observe because it always appears close to the Sun in Earth's sky. Only a few spacecraft have explored it closely, including Mariner 10 and NASA's MESSENGER mission.
The joint European Space Agency and JAXA BepiColombo mission is expected to enter orbit in late 2026 and will study the planet's surface, interior, magnetic field, and thin atmosphere in much greater detail.
The Solar System’s Extreme Inner World
Mercury is a planet of extremes. It races around the Sun faster than any other planet, experiences scorching days and freezing nights, and preserves ancient scars from the earliest days of the Solar System.
Though small and difficult to observe, Mercury offers important clues about how rocky planets form and evolve close to their stars.